Stress sensing devices and methods

ABSTRACT

Embodiments relate to stress sensors and methods of sensing stress. In an embodiment, a stress sensor comprises a vertical resistor. The vertical resistor can comprise, for example, an n-type resistor and can have various operating modes. The various operating modes can depend on a coupling configuration of terminals of the resistor and can provide varying piezo-coefficients with very similar temperature coefficients of resistances. Comparisons of resistances and piezo-coefficients in differing operating modes can provide a measure of mechanical stresses acting on the device.

RELATED APPLICATION

This application is a continuation of application Ser. No. 13/556,633,filed Jul. 24, 2012, which in turn is a continuation of application Ser.No. 12/714,605 filed Mar. 1, 2010, now U.S. Pat. No. 8,240,218 issuedAug. 14, 2012, each of which is hereby fully incorporated herein byreference.

TECHNICAL FIELD

The invention relates generally to stress sensing devices and moreparticularly to integrated circuit mechanical stress sensors utilizingvertical resistors.

BACKGROUND

Mechanical stress changes the electronic parameters of micro-electronicdevices. Circuits that depend on these parameters or absolute valuesthereof, such as Hall sensors, constant current sources and band-gapregulated reference voltages, can suffer from a stress drift. The driftof these circuits can be reduced by measuring the mechanical stress andcompensating the resulting drift for this measured stress change.

Stress sensors can be made by comparing two different diffused orimplanted resistors, such as a lateral p-type resistor with a lateraln-type resistor or a lateral n-type resistor with a vertical n-typeresistor. The former can suffer from different temperature coefficientsbetween p- and n-type resistances. While the latter has much bettermatching of temperature coefficients, there is still a notabledifference in these coefficients between vertical and lateral n-typeresistances. In practice, different n-wells for lateral and verticalresistors are typically used in CMOS technology such that thetemperature coefficient, which depends on the doping level, is notidentical. Additionally, a significant technology spread betweendifferent doping levels usually has to be contended with.

SUMMARY

Embodiments relate to stress sensing devices and methods. In anembodiment, a stress sensing device comprises an active semiconductorlayer; and a plurality of contacts for the active semiconductor layer,the plurality of contacts spaced apart from one another, wherein thestress sensing device is configured to sense a mechanical stress fromvertical components in the active semiconductor layer of currentsapplied to at least one of the plurality of contacts.

In an embodiment, a stress sensing device comprises an active layer; atleast two contacts arranged in a first surface of the active layer; anda buried layer coupled to a second surface of the active layer oppositethe first surface, wherein the at least two contacts, the active layerand the buried layer comprise n-elements, and a mechanical stress isrelated to a portion of a current that flows in the active layer from atleast one of the at least two contacts.

In an embodiment, a stress sensing device comprises an activesemiconductor region; and at least three contacts for the activesemiconductor region, the at least three contacts spaced apart from oneanother and configured to apply currents with vertical components toflow through the active semiconductor region, wherein the stress sensingdevice is configured to sense mechanical stress based on the appliedcurrents.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 depicts a side view cross-sectional block diagram of a stresssensor device according to an embodiment.

FIG. 2A depicts a top view block diagram of a stress sensor deviceaccording to an embodiment.

FIG. 2B depicts a top view block diagram of a stress sensor deviceaccording to an embodiment.

FIG. 2C depicts a top view block diagram of a stress sensor deviceaccording to an embodiment.

FIG. 2D depicts a top view block diagram of a stress sensor deviceaccording to an embodiment.

FIG. 2E depicts a top view block diagram of a stress sensor deviceaccording to an embodiment.

FIG. 2F depicts a top view block diagram of a stress sensor deviceaccording to an embodiment.

FIG. 3A is a side view cross-sectional diagram of current flow in astress sensor device according to an embodiment.

FIG. 3B is a side view cross-sectional diagram of current flow in astress sensor device according to an embodiment.

FIG. 3C is a side view cross-sectional diagram of current flow in astress sensor device according to an embodiment.

FIG. 3D is a side view cross-sectional diagram of current flow in astress sensor device according to an embodiment.

FIG. 4A depicts a top view block diagram of an L-layout stress sensordevice according to an embodiment.

FIG. 4B depicts a top view block diagram of an L-layout stress sensordevice according to an embodiment.

FIG. 5A depicts a side view cross-sectional diagram of a stress sensordevice according to an embodiment.

FIG. 5B is a top view block diagram of a stress sensor device accordingto an embodiment.

FIG. 5C is a top view block diagram of a stress sensor device accordingto an embodiment.

FIG. 5D is a top view block diagram of a stress sensor device accordingto an embodiment.

FIG. 5E is a top view block diagram of a stress sensor device accordingto an embodiment.

FIG. 5F is a top view block diagram of a stress sensor device accordingto an embodiment.

FIG. 5G is a top view block diagram of a stress sensor device accordingto an embodiment.

FIG. 5H is a top view block diagram of a stress sensor device accordingto an embodiment.

FIG. 6 is a side view cross-sectional block diagram of a stress sensordevice according to an embodiment.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

Embodiments relate to stress sensors and methods of sensing stress. Inan embodiment, a stress sensor comprises a vertical resistor. Thevertical resistor can comprise, for example, an n-type resistor and canhave various operating modes. The various operating modes can depend ona coupling configuration of terminals of the resistor and can providevarying piezo-coefficients with very similar temperature coefficients ofresistances. Comparisons of resistances and piezo-coefficients indiffering operating modes can provide a measure of mechanical stressesacting on the device.

Referring to FIG. 1, a stress sensor comprising a resistive device 100is depicted. In an embodiment, device 100 comprises at least threecontacts or terminals, providing for operation of device 100 in at leasttwo operating modes having different piezo-coefficients. Because theeffective piezo-coefficient of the resistance depends on the operatingmode, a comparison of resistance values from different operating modesis a unique measure of the mechanical stress affecting the device 100.

In FIG. 1, device 100 is laterally symmetrical as depicted on the pageand comprises five contacts: three center contacts 102, 104 and 104′ andtwo outer contacts 108 and 108′. The contacts 102, 104, 104′, 108 and108′ are formed in an epitaxial layer 110 with the center contacts 102,104 and 104′ separated from the outer contacts 108 and 108′ by twoisolation regions 106 and 106′. In an embodiment, device 100 comprisesan n-buried layer below epitaxial layer 110, which itself is a lown-doped region, which establishes a short at the bottom of layer 110discussed in more detail below. Device 100 also comprises a trench 112(FIG. 2) to isolate device 100 from the rest of the die.

In one embodiment, contacts 102, 104 and 104′ are shallow n+S/Ddiffusions having a depth of about 0.2 μm and a width of about 2.5 μm.Contacts 102 and 104, and 102 and 104′, are spaced apart respectively byabout 2.5 μm in an embodiment. Isolation regions 106 and 106′ are 1.3μm-deep p-tubs spaced apart from contacts 104 and 104′, respectively, byabout 0.7 μm in an embodiment, while outer contacts 108 and 108′ aren-CMOS wells having a depth of about 1 μm in an embodiment, spaced apartfrom isolation regions 106 and 106′, respectively, by about 0.6 μm andfrom an end of epitaxial layer 110 by about 0.8 μm. The variousdimensions, spacings and configurations are merely exemplary of anembodiment and can vary in other embodiments, as is discussed in moredetail below.

Referring also to FIG. 2, a plurality of exemplary embodiments aredepicted in which the size, number, configuration and/or othercharacteristics of contacts 102, 104, 104′, 108 and 108′ and isolationregions 106 and 106′ varies. These and other variations providedifferent operating modes, depending on the number of operating modes(related to the number of contacts) and configurations of the contactsthat affects the resistance. In FIG. 2A, device 200A is similar todevice 100 in that it comprises contacts 102, 104, 104′, 108 and 108′ aswell as isolation regions 106 and 106′ respectively spaced apart in asimilar manner. In FIG. 2B, contact 102 is differently sized. In FIG.2C, contacts 102, 104 and 104′ are similarly sized though smaller thanin device 200 a of FIG. 2A. In FIG. 2D, device 200 d comprisesadditional center contacts 114, 116, 116′, 118, 120 and 120′. In FIG.2E, device 200 e is similar to device 200 c of FIG. 2C except that theconfiguration of isolation regions 106 and 106′ has changed. In FIG. 2F,device 200 f comprises a single center contact 102 and has a similarisolation region 106 and 106′ configuration as that of device 200 e inFIG. 2E.

In devices 100 and 200 a-f, the overall resistance in each devicedepends on the mechanical stress affecting the device. Measuring themechanical stress, however, is difficult, in part because it can dependon temperature. In embodiments, therefore, the device can be operated inmultimode modes depending on coupling arrangements of the contacts. Itis desired that a larger portion of the current flows in a verticaldirection in one of the modes versus the other. Then, the resistances inthe multiple modes can be compared, with temperature factored outbecause it is constant across the multiple modes. In an embodiment, apercentage change of resistance between at least two operating modes isdetermined according to ΔR/R, which is equal to the piezo-coefficienttimes the mechanical stress affecting the device.

The piezo-coefficient is related to direction of current flow in thedevice, generally having horizontal and vertical components. Referringagain to FIGS. 1 and 2A as well as FIG. 3A, when current is injectedinto one or more of the contacts of device 100 or 200 a, current flowsgenerally into and through epitaxial layer 110. The current flows bothhorizontally and vertically, with different ratios of horizontal andvertical components throughout epitaxial layer 110 providing aneffective piezo-coefficient. In general herein, calculations arerestricted to in-plane stress, ignoring vertical stress, which is arealistic assumption for real integrated circuit (IC) packages.

FIG. 3A relates to a first operating mode and the embodiment depicted inFIGS. 1 and 2A, though only one half, the right half as depicted on thepage of FIG. 1, is shown in FIG. 3A. In this operating mode, centercontacts 102, 104 and 104′ are held at 1 V while outer contacts 108 and108′ are grounded. As can be seen in FIG. 3A, the current flowsprimarily from center contacts 102 and 104 vertically downward throughepitaxial layer 110 to the buried layer, then outward in the low ohmicburied layer and upward to the outer contact 108. A portion of thecurrent, however, is shunted directly from center contacts 102 and 104to outer contact 108, as can be seen in layer 110 below isolation region106. Further, the current lines flowing mainly vertically to the buriedlayer show some spreading, indicating both horizontal and verticalcomponents. This blend of horizontal and vertical components leads to amixing of respective piezo-coefficients. In FIG. 3A, and assuming thehorizontal axis of device 100 is aligned to the [100] direction of thesilicon crystal and the vertical axis to the [001], the resistance isequal to:

$R = {R_{0}\left( {1 + {\frac{43.83\%}{GPa} \cdot \left( {\sigma_{11} + \sigma_{22}} \right)} - {\frac{9.34\%}{GPa} \cdot \left( {\sigma_{11} - \sigma_{22}} \right)}} \right)}$where GPa is giga-Pascals and equal to 10^9 N/m², with N being Newtonsand m, meters.

In FIG. 3B, current flowlines in a second operating mode of device 100are shown. In this operating mode, contact 104 (and contact 104′, notdepicted given that only the right symmetrical half of device 102 isshown in FIGS. 3A-D) is left floating, while contact 102 is held at 1 Vand outer contact 108 (and 108′, not depicted) is grounded. Comparedwith the current flowlines in FIG. 3A, the flowlines in FIG. 3B have alarger horizontal component, and more current is shunted from contact104 to contact 108. The floating contact 104 (and 104′) lead to a shortcircuiting effect. The smaller center contact 102 leads to morespreading of the current flowlines, also visible in FIG. 3B, whichincreases the ratio of horizontal components. It follows that thepiezo-resistance approaches the lateral resistance, which has a smallerpiezo-coefficient than the vertical resistance. The resistance for theoperating mode of FIG. 3B can be expressed as:

$R = {R_{0}\left( {1 + {\frac{31.00\%}{GPa} \cdot \left( {\sigma_{11} + \sigma_{22}} \right)} - {\frac{22.14\%}{GPa} \cdot \left( {\sigma_{11} - \sigma_{22}} \right)}} \right)}$

Current flowlines in device 100 in a third operating mode is depicted inFIG. 3C, in which contact 102 is floating, contact 104 (and 104′) is at1 V and contact 108 (and 108′) is grounded. The piezo-resistive behaviorcan be expressed as:

$R = {R_{0}\left( {1 + {\frac{39.64\%}{GPa} \cdot \left( {\sigma_{11} + \sigma_{22}} \right)} - {\frac{13.54\%}{GPa} \cdot \left( {\sigma_{11} - \sigma_{22}} \right)}} \right)}$

A fourth operating mode of device 100 is depicted in FIG. 3D. In thisoperating mode, contacts 102 and 104 (and 104′) are floating, contact108′ (not shown in FIG. 3D) is at 1 V and contact 108 is grounded. Thepiezo-resistance in this mode is:

$R = {R_{0}\left( {1 + {\frac{47.37\%}{GPa} \cdot \left( {\sigma_{11} + \sigma_{22}} \right)} - {\frac{5.46\%}{GPa} \cdot \left( {\sigma_{11} - \sigma_{22}} \right)}} \right)}$

In the four operating modes of FIGS. 3A-D, the piezo-drift of resistancevalues with respect to the sum of the in-plane stresses ranges from47%/GPa to 31% GPa, differing by a factor of more than 1.5. Given thesignificant difference, these measurements can be used to extract themechanical stress. As previously mentioned, this can be done in oneembodiment by operating device 100 in at least two different operatingmodes and comparing the resistances measured in each. Even at zerostress, the resistances in different operating modes will not beidentical, but this is unimportant in embodiments because the ratio ofzero stress resistances shows a small spread due to layout andmanufacturing tolerances. For coarse stress sensors, this spread may beneglected in embodiments, with a measured ratio compared with a nominalvalue obtained theoretically or by characterization of typicalproduction lots. For more accurate stress sensors, the ratio can bemeasured, e.g., on wafer level prior to packaging, and the ratio thenstored in on-chip memory, such as EEPROM, fuses or some other form.During operation in the field, the measured ratio can be compared withthe stored ratio, with any deviation due to a stress difference.

So far, differences of normal stress components have been neglected.Assuming a second device rotated around [001] by 90 degrees, thehorizontal axis in FIGS. 3A-D would then correspond to [010] instead of[100]. The piezo-resistance for this device is obtained in an embodimentby exchanging σ11 with σ22. Thus, the sign of the difference termchanges, so the differences terms can be canceled when two devices withperpendicular orientations in layout are used and connected in series orparallel, a so-called “L-layout,” an embodiment of which is depicted inFIG. 4. In FIG. 4, device 400 depicts different mode series connectionsof devices 200 f, though any device 100, 200 a-f or some otherappropriate stress sensor device can be used in other embodiments. Theratio of R1/R2 is indicative of mechanical stress in device 400.

As discussed above, temperature concerns fall away because a singledevice is used in two operating modes, and any temperature dependencecancels out in the ratio of resistance values. In an embodiment, this isachieved using constant voltage biasing, not constant current biasing,because the latter changes the potentials versus temperature and stressand therefore also the exact geometry of the active area due tomodulation of space charges layers.

Embodiments discussed above can utilize n-buried layer technology,though other embodiments without such a layer can also be implemented.If there is no short at the bottom of epitaxial layer 110, differentratios of horizontal to vertical current flow can be achieved by varyingthe spacing between contacts. Referring to FIG. 5A, if current flowsbetween two contacts C1 and C3 that are spaced apart by a distancegreater than the depth of active volume 510, horizontal components willdominate vertical components. If the contacts are closer than the depth,such as for C1 and C2, the current flowlines take a semicircular path,with horizontal and vertical components closer to equal. Device 500 alsocomprises isolation trenches or cups 506.

Another view is shown in FIG. 5B. Device 500 b comprises three contactsC1, C2 and C3 arranged in, in one embodiment, a low n-doped region 510.Contacts C1 and C2, and C2 and C3 are separated, respectively, byisolation trenches 506′ and 506. A p-substrate 512 isolates an outerportion of device 500. Ratios of resistances between C2-C1 and C3-C1, aswell as C2-C3 and C3-C1, are indicative of mechanical stress.

Another configuration, shown as device 500 c, is depicted in FIG. 5C inwhich the configuration of contact C2 is altered. The same variousratios of resistance are available in device 500 c as discussed abovewith respect to device 500 b. Another embodiment is depicted in FIG. 5D,in which isolation trenches 506 and 506′ are omitted.

In the embodiment of FIG. 5E in which contact C2 is small relative tocontacts C1 and C3, the ratio of resistance between C2-C1 and C2-C3 isgenerally independent of mechanical stress. The ratio of resistancebetween C2-C1 and C3-C1 depends on stress. Because the distance of C3-C1is larger than that between C2-C1, it would lead to a larger resistancebetween C3-C1 than between C2-C1, if all contacts C1, C2, C3 are thesame size. If, for example, C2 is made smaller, the resistance of C2-C1increases. Thus, it can be an advantage in embodiments to reduce thesize of C2 so that at zero stress the resistance between C2-C1 is equalto the resistance between C3-C1.

In FIG. 5F, the resistance between C1 and C2 is lower than that betweenC1-C3 or C2-C3, making this configuration less attractive. The ratio ofresistances between C2-C1 and C3-C1 is indicative of mechanical stress,however.

Another configuration is depicted in FIG. 5G. Device 500 g of FIG. 5ghas two blocks of contacts, C and B. Though generally depicted as havingthe same size and number of contacts, this need not be the case in otherembodiments. A first resistance can be measured between C22 and B22, orany other pair of contacts. A second resistance can be measured betweenall C contacts, shorted via switches, and all B contacts, also shortedvia switches. The ratio of both resistances is indicative of mechanicalstress in device 500 g.

FIG. 5H depicts a device 500 h with rotational symmetry. The resistancebetween C2-C1 can be designed to be similar to the resistance betweenC3-C1. The ratio of resistances between C2-C1 and C3-C1 is indicative ofmechanical stress, as is the ratio between C2-C1 and C2-C3. Sensitivityto mechanical stress is all the larger if C3 is large is C1 and C2relatively smaller. In an embodiment, 510 is an n-doped epitaxial layerand 512 a p-doped isolation layer, as in other embodiments. An advantageof the embodiment of FIG. 5H is that an L-layout, such as is depicted inFIG. 4, is intrinsic, eliminating the need for a second device 500 tocancel the terms σ11-σ22 in the resistance.

In various embodiments, the stress sensing devices comprises switches inorder to couple and decouple the contacts to achieve differing operatingmodes. In an embodiment, such as the one depicted in FIG. 6, theswitches can comprise MOS switches integrated into the stress sensingdevice. In device 600, current is supplied via contact C, which iscoupled to a highly doped p Source/Drain diffusion, p+S/D. If highpotentials are supplied to G1 and G2, no inversion channel can developbelow these gates. If enough voltage is applied at C, the pn-junctionbetween the p+S/D and the n-CMOS-well is forward biased, and current isinjected into the n-epitaxial layer flowing downward to the buriedlayer, then up again at the right side, where an ohmic contact isapplied at B via an n+S/D diffusion. By applying low potentials at G1and/or G2, channels develop below the gates, and the current is spreadout over a larger area of device 600. Thus, the current has morevertical portions, shown in solid arrows, and the resistance of device600 has a larger piezo-coefficient in this operating mode. In anembodiment, the p+S/D diffusions at the left and right are omitted,though an advantage of these regions is that if G1 and G2 are off, thecontacts have no short circuiting effect on the current because they arenot conducting. A disadvantage is added complexity, a special device andthe fact that there is always a forward diode voltage drop in additionto the piezo-resistance, which should be extracted.

In operation, embodiments of the invention can be used to monitorassembly line and processes and to detect changes or flaws therein.Additionally or alternatively, embodiments can be mounted in modules andused to read out resistance ratios as discussed in order to detect diecracks in overmolding. Other uses, configurations and implementationsare also contemplated.

Various embodiments of systems, devices and methods have been describedherein. These embodiments are given only by way of example and are notintended to limit the scope of the invention. It should be appreciated,moreover, that the various features of the embodiments that have beendescribed may be combined in various ways to produce numerous additionalembodiments. Moreover, while various materials, dimensions, shapes,implantation locations, etc. have been described for use with disclosedembodiments, others besides those disclosed may be utilized withoutexceeding the scope of the invention.

Persons of ordinary skill in the relevant arts will recognize that theinvention may comprise fewer features than illustrated in any individualembodiment described above. The embodiments described herein are notmeant to be an exhaustive presentation of the ways in which the variousfeatures of the invention may be combined. Accordingly, the embodimentsare not mutually exclusive combinations of features; rather, theinvention may comprise a combination of different individual featuresselected from different individual embodiments, as understood by personsof ordinary skill in the art.

Any incorporation by reference of documents above is limited such thatno subject matter is incorporated that is contrary to the explicitdisclosure herein. Any incorporation by reference of documents above isfurther limited such that no claims included in the documents areincorporated by reference herein. Any incorporation by reference ofdocuments above is yet further limited such that any definitionsprovided in the documents are not incorporated by reference hereinunless expressly included herein.

For purposes of interpreting the claims for the present invention, it isexpressly intended that the provisions of Section 112, sixth paragraphof 35 U.S.C. are not to be invoked unless the specific terms “means for”or “step for” are recited in a claim.

The invention claimed is:
 1. A stress sensing device comprising: asemiconductor substrate; a layer arranged within the semiconductorsubstrate, a plurality of contacts arranged on a first side of thelayer, wherein a first resistive element formed within the layer betweena first pair of contacts of the plurality of contacts; a secondresistive element formed within the layer between a second pair ofcontacts of the plurality of contacts; wherein the first resistiveelement and the second resistive element show a substantially identicalresponse to temperature changes, and a ratio of the first and secondresistive elements varies with applied mechanical stress.
 2. The stresssensing device according to claim 1, wherein the contacts of the firstpair of contacts are arranged closer to each other than contacts of thesecond pair of contacts.
 3. The sensor device of claim 1, wherein thecontacts are contact diffusions.
 4. The stress sensing device accordingto claim 1, wherein the first pair of contacts comprises at least onecontact of the plurality of contacts that is different from the contactsof the second pair of contacts.
 5. The stress sensing device accordingto claim 1, wherein the contacts of the first pair of contacts aredifferent from the contacts of the second pair of contacts.
 6. Thestress sensing device according to claim 1, wherein the first resistiveelement is different from the second resistive element.
 7. The stresssensing device according to claim 1, wherein the layer is n-doped orp-doped.
 8. The stress sensing device according to claim 1 furthercomprising isolating trenches or cups.
 9. The stress sensing deviceaccording to claim 1, wherein the layer has a depth, and whereincontacts of the first pair of contacts are spaced apart at a distancesmaller or substantially equal to the depth, while contacts of thesecond pair of contacts are spaced at a distance substantially largerthan the depth.
 10. The stress sensing device according to claim 9,wherein for a current flowing through the second resistive element, ahorizontal current contribution will dominate over a vertical currentcontribution.
 11. The stress sensing device according to claim 10,comprising a low ohmic layer in ohmic contact to the layer and arrangedon the opposite side of the first side.
 12. The stress sensing deviceaccording to claim 9, wherein for a current flowing through the secondresistive element, a vertical current contribution will dominate over ahorizontal current contribution.
 13. The stress sensing device accordingto claim 1, wherein an isolating region is arranged on the first side ofthe layer between contacts of the first pair of contacts or betweencontacts of the second pair of contacts.
 14. A sensor device comprising:a layer; and at least three contacts spaced apart from one another inthe layer, the at least three contacts being coupleable in a firstconfiguration for a first operating mode of the sensor device in which acurrent in the layer has a first ratio of horizontal to verticalcomponents with respect to a die surface, and in a second configurationdifferent from the first for a second operating mode of the sensordevice in which a current in the layer has a second ratio of horizontalto vertical components, wherein a ratio of a resistance between at leasttwo of the contacts in the first operating mode and a resistance betweenat least two of the contacts in the second operating mode is related tomechanical stress in the sensor device.
 15. The sensor device of claim14, further comprising at least one isolation region, the at least oneisolation region arranged between first and second ones of the at leastthree contacts or arranged between second and third ones of the at leastthree contacts.
 16. The sensor device of claim 15, wherein the at leastone isolation region comprises isolation wells.
 17. The sensor device ofclaim 1, further comprising a buried layer under an active layer. 18.The sensor device of claim 1, wherein the at least three contacts arearranged symmetrically about an axis in the layer.
 19. The sensor deviceof claim 1, wherein at least one of the at least three contacts isselected from the group consisting of: an n+S/D diffusion and an n-CMOSwell.
 20. The sensor device of claim 1, wherein the sensor device isrotationally symmetrical.
 21. A method of sensing mechanical stressusing the stress sensing device of claim 1, the method comprisingcomparing a resistance of the first resistive element with a resistanceof the second resistive element, wherein the comparison result indicatesan amount of mechanical stress.